Creatures roaming the balmy swamps and forests of the prehistoric Arctic survived the long dark season by switching diets, according to a new study.

Instead of migrating south or hibernating for the winter like most animals today, they endured, foraging in dormant forests through the long polar gloaming.

The research, which was led by Jaelyn Eberle, an assistant professor at the University of Colorado in Boulder, appears in the latest edition of the journal Geology.

Canada's Ellesmere Island is a frigid tundra. But 53 million years ago it resembled a swamp, teeming with plants and crowded with alligators, turtles and tapirs. Even lemur-like creatures swung from the trees.

Scientists have long wondered how the animals survived the long winter.

Though temperatures didn't often get below freezing, plants must have gone into hibernation during the six months of darkness.

New evidence extracted from the teeth of the hippopotamus-like Coryphodon suggests they subsisted on leaf litter, decaying fruits and twigs through the winter.

Eberle and colleagues sampled carbon isotopes from the teeth of nine Coryphodons, three tapirs and two rhinoceros-like brontotheres, all of which lived on ancient Ellesmere.

The results show the plant-eaters lived off of abundant flowering plants, leaves and greenery in the spring and summer months. But in the winter they switched to eating dead and dying plant matter and fungi.

'Stranger than fiction'

"People first discovered alligators in the high Arctic in 1975, and I've been going up there since the 1990's," says Eberle. "But we never thought to stop and ask 'why?' or 'how?' How were these animals able to survive up there - what made them special?"

Dead plant matter is mostly low in nutritional value, so the animals must have compensated each winter by eating huge amounts of food, and thoroughly digesting it.

"In a way this is a case of truth being stranger than fiction. We don't have an ecosystem like this extinct ecosystem. It's amazing how resistant to changes in light these animals can be," says Christopher Beard of the Carnegie Museum of Natural History in Pittsburgh, Pennsylvania.

He says Eberle's theory is "a persuasive, interesting hypothesis," though he stresses that it's still far from certain.

As global warming affects the Arctic, Eberle expects animals will again move north. But she says it's unlikely alligators or hippos will soon return.

The early Eocone climate of Ellesmere was drastically warmer than present day, averaging temperatures 25°C more back then.